Laser imaging and ranging system using two cameras

ABSTRACT

A pulsed laser illumination based two camera ranging and imaging system which requires only a single axis optical aperture and is therefore suited to use on aircraft and other space and configuration limited apparatus is disclosed. The invention uses intensity coded partial images of the distant target with the partial images being obtained by a distance segregating optical modulation arrangement which separates pixel portions according to their arrival time and thereby their distance of travel from the target. The described system uses pixel by pixel processing of the segregated images as accomplished in an image array processor or general purpose computer, for example.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

REFERENCE TO RELATED APPLICATION

This application is somewhat related to application Ser. No. 07,677,896titled LASER IMAGING AND RANGING SYSTEM USING ONE CAMERA that is filedin the name of the same inventors and filed of even date herewith.

BACKGROUND OF THE INVENTION

This invention relates to the field of optical range mapping of thenontriangulation type.

In activities as diverse as midair refueling, automatic targetrecognition, robotic vision, space docking, industrial inspection andcustomized equipment design, for example, there is need for precisemappings of range measurements. Often such measurements are overdistances in excess of that accommodated by conventional graduatedtension member measurement or impractical of such measurement.

The term mapping in this description includes the concept of rangedetermination for a multiplicity of different points located on thedistal target object and therefore is inclusive of aspects relating toshape and size of the target object.

In the past, such techniques as triangulation using optical images takenfrom two different viewpoints, or the time for energy propagation to andfrom points on a distal target object have been employed foraccomplishing range mapping measurements. In range mapping systems,however, there is often a need for such speed and accuracy andoperational simplicity in determining range maps of a distal target asto preclude the use of previous measurement techniques. Thetriangulation range determination, for example, as is often used incameras and in artillery measurement periscopes is slow and oftencumbersome in range mapping accomplishment and somewhat difficult toperform automatically as a computer or electronics system operatingfunction.

Since triangulation usually involves two superimposed or split fieldimages for indication, it often requires the conclusion of a humaninterpreter to reliably complete the measurement. Another example oftriangulation impracticability is to be found in the field of airborneweaponry wherein it is often difficult to separate a plurality ofoptical apertures by a meaningful distance on the limited confines of anairframe--and yet there is a strong need for quick and accuratemeasurement of range maps for use in rocketry and other weapons relatedfunctions.

Since many current uses for a range mapping system impose requirementsof nonhuman interpretation, small physical size, the use of a singleoptical aperture, and accuracy in both the near field and far fieldenvironment, there is need in the art for an improved range mappingsystem such as the present "Laser Imaging and Ranging Systems Using TwoCameras" . Parenthetically, in view of this name and accomplishedfunction, the system of the present invention can be convenientlyreferred to by way of the acronym LIMAR/2 which is based on the first orfirst and second letters of the words appearing in the name. The presentdocument hereby incorporates by reference the above referred-to onecamera laser imaging and ranging system patent document.

The patent art includes several examples of range mapping systemsincluding laser operated systems which are of interest with respect tothe present invention. Included in these patent examples is U.S. Pat.No. 3,409,369, issued to G. W. Bickel and concerning a laser radarsystem which operates on the Doppler velocity measuring principle usingtwo different transmitted frequencies in order to obtain a convenientlylow difference frequency. Since the Bickel apparatus is principallyconcerned with the Doppler operating concept and target velocitymeasurement, the present invention is readily distinguished therefrom.

Also included in these prior patents is U.S. Pat. No. 3,465,156 issuedto C. J. Peters and concerned with a laser communication system whichemploys a narrow band noise cancellation technique with the transmittedlaser beam divided into two different paths. The Peters inventionreceived laser light is also split into two beams one of which containsvideo signal modulation and both of which contain noise components.Since the Peters apparatus is concerned with a laser communicationsystem, the present invention range mapping concepts are readilydistinguished.

Also included in these prior patents is U.S. Pat. No. 3,504,182 issuedto V. F. Piezzurro et al and concerned with an optical communicationsystem. In the Piezzurro et al communication system scanning by the beamemitted from one station is used to "acquire" the scanning pattern of asecond station in order that the two stations can lock-on and be readyto transmit and receive information. Distinctions between the presentinvention and the communications system of Piezzurro et al are readilyapparent.

Also included in this art is the laser radar mapping system of F. K.Knight et al reported in Applied Optics, Vol. 28, pp. 2196-2198, June1989. In this system, use is made of a streak camera to obtain rangemaps of objects at a distance. The Knight et al apparatus does not teachthe use of an electromagnetic shutter encoding as in the presentinvention.

Also included in this art is the U.S. Pat. No. 4,515,472 of A. B. Welchwhich is concerned with an agile receiver for a scanning laser radarsystem. The Welch apparatus uses a receiver frequency adjustmentarrangement in order to rapidly acquire, recognize, track and performsimultaneous guidance functions for a multiplicity of weapons against amultiplicity of targets. In view of this purpose and functioning of theWelch apparatus, distinction from the present invention range mappingsystem is easily discerned.

SUMMARY OF THE INVENTION

In the present invention short pulsed laser radiation is projected ontoa target object and the returning scattered light captured and modulatedby an electro-optic system and then split into two separate andcomplementary partial images at two optical to electrical transducercamera devices. In the partial images a returning signal is opticallymodulated with a time dependent range indicating signal. This modulationenables decoding of the pixel intensity in the corresponding images ofthe two cameras to derive a range indication. The decoding may employ animage processing system. The invention provides a numerical map of rangeindication that is registered with an optical image of the systemoutput.

An object of the present invention is therefore to provide a singleoptical aperture laser range map measuring system.

Another object of the invention is to provide a range measuringarrangement in which the range characteristic information is coded intothe form of pixel intensity in an optical image.

Another object of the invention is to provide a range mapping system inwhich an image of the range mapped target is also available.

Another object of the invention is to provide a range mapping system inwhich the intensity coded information of two range map determiningpartial images is also usable for reconstructing a full image of therange mapped target.

Another object of the invention is to provide a range map measuringsystem in which the range communicating signals are preserved in theform of pixel intensity ratio modulation without reliance on absolutevalues of pixel intensity.

Another object of the invention is to provide a range indicating systemin which two partial images from single axis views of the range mappedtarget are utilized.

Another object of the invention is to provide for the presentation oftwo-dimensional and three-dimensional image information in a fullycompatible format.

It is another object of the invention to provide a single apertured twocamera range mapping system.

It is another object of the invention to provide a single aperturedrange mapping system in which the range indicating data is stored in theform of two partial images of the range determined target.

It is another object of the invention to provide a range mapping systemwhich is especially convenient for use on aerospace and autonomousvehicles.

It is another object of the invention to provide a time of propagationrange measurement system which employs simplified electro-opticalswitching.

It is another object of the invention to provide a range mapping systemin which noise reduction through the use of signal time averaging may beaccomplished.

It is another object of the invention to provide a time of propagationrange determining system which can be accomplished without use of astreak camera.

It is another object of the invention to provide a range mapping systemwhich can be accomplished without the use of moving parts.

These and other objects of the invention are achieved by a singleoptical aperture target range determination system having pulsed opticalenergy generating laser means for illuminating said target via saidoptical aperture; optical modulator means for dividing the reflectedoptical signal returning to said aperture from said pulse illuminatedtarget into two time-of-arrival proportioned range coded optical signalcomponents; means for generating electrical signals indicative of themagnitude of said range coded first and second component opticalsignals; and means for recording electrical signals in pixel organizedarray.

Additional objects and features of the invention will be understood fromthe following description and claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a range mapping system in accordancewith the invention.

FIG. 2 shows the concept of image pixel intensity modulated storage usedin the FIG. 1 system.

FIG. 3 shows the modulating waveform used with the FIG. 1 system.

FIG. 4 shows elements of a modulation waveform generating circuit usablein the FIG. 1 system.

FIG. 5 shows an element arrangement for achieving the optical signalmodulation employed in the FIG. 1 system.

DETAILED DESCRIPTION

FIG. 1 in the drawings shows a two camera embodiment of a laser imagingand ranging system that is in accordance with the present invention. Inthe FIG. 1 system, a distal target 100 is illuminated by light from alaser 104 by way of an optical system that is generally indicated at110. The returning reflected light from the target 100 is ultimatelyreceived in a pair of video cameras 101 and 102 in FIG. 1 where it istransduced into a pair of electrical signals that are conducted alongthe paths 103 and 105 to a digital image processing system 106.

In the FIG. 1 optical system coherent light emission from the laser 104is communicated along the path 114 by way of the mirrors 112 and 115 tothe path 116 which terminates in the distal target 100. Light reflectedfrom the target 100 is communicated by the path 120 to the concave andconvex mirrors 118 and 122 for reflection through the mirror aperture119 to 10 the optical filter elements 124 and 126 and thence to theelectro-optical polarization modulating element 128. The filter 124 inthis optical system is of the optical bandpass type and restricts thelight transmitted from the convex mirror 122, that is light along thepath 133, to a narrow spectral band which includes the wavelength of thelaser 104. The filter element 124 additionally restricts the light alongthe path 133 to components which are polarized in a single plane-such asthe plane perpendicular to the FIG. 1 drawing, for example.

The electro-optical element 128, which may be of the Kerr cell orPockels cell type, accomplishes a controllable degree of polarizationrotation in response to electrical signals received along the path 134.Polarized light that is rotated in accordance with the modulation signalreceived on the path 134 emerges from the electro-optic modulatingelement 128 along the path 135 whereupon it enters the polarizing beamsplitting cube 131 and is divided into two complementary amplitudeoptical signals along the paths 137 and 139. These optical signals arereceived on the optical to electrical transducer retina portions of thevideo cameras 101 and 102 where conversion to electrical signals thatare communicated along the paths 103 and 105 to the microprocessordigital image processing system 106 occurs. In the image processingsystem 106, the intensities of corresponding pixels of the imagesreceived at the cameras 101 and 102 are compared for a determination ofrange of the distal target 100. A time dependent proportioning of thereturning light signal on the path 133 into signals along paths 137 and139 is achieved in the FIG. 1 system in order to accomplish the desiredrange map.

The electro-optical polarization modulating element 128 is connected tothe Driver-Pulse Generator Circuitry (DPGC) 108 by the path 134 forcommunication of the driving signals generated by the driver-pulsegenerating circuit 108. The DPGC 108 in turn receives control signalsalong the path 140 from the input-output circuit portion 142 of themicrocomputer digital image processing system 106 as well assynchronizing pulses from the laser 104 along the path 132.

A conventional computer input-output circuit card such as the analogDevices Model RTU-800 or 815 may be used for the input-output circuitportion 142 of the microcomputer 106 to accomplish the control signalgeneration. The microcomputer 106 can be one of the many such computersnow available commercially including an IBM-AT personal computer as ismanufactured by IBM Corporation of New York state, U.S.A. Imageprocessing boards such as the ITEX type 100 processors may be receivedat 144 and 146 in the computer 106. The output of the image processingboards 144 and 146 may be coupled to a pair of memories where pixel bypixel storage or other storage arrangements are used to enablemathematical processing of the data for range mapping. This memory maybe included in the processor 106 or located external thereto.

Referring to FIG. 4 the DPGC 108 of FIG. 1 is shown to be comprised of asignal delay generator 402 and a high voltage signal generator. Thesignal delay generator 402 may be, for example, a Stanford ResearchSystems Inc. Model DG 535 Pulse Generator or similar apparatus. TheStanford generator is made by Stanford Research Systems Inc. of PaloAlto, Calif.

The high voltage signal generator 406 is comprised of a capacitorcharging source, the resistor 416 and the source of high voltage DCenergy 428, together with a fast electronic controlled switch 400 and apulse amplifier 404. The switch 400 may be comprised of a transistorsuch as the IRFPG 50 power field effect switching transistor sold byInternational Rectifier Corporation or may be a vacuum tube or solidstate device capable of operating at the five hundred volt or moresignal levels described herein for the electro-optical polarizationmodulating element 128. A switching transistor system of this type isdisclosed in the article "Nanosecond Switching Using Power FET's" by R.J. Baker et al appearing in Review of Scientific Instruments, volume 61,page 2211 (1990) which is hereby incorporated by reference herein.

The pulse amplifier 404 in FIG. 4 is controlled by the generator 402 andprovides signals suitable for the FET gate or vacuum tube grid electrodeor other input of the switching device 400. Depending upon thecapabilities of the generator 402 and the requirements of the switchingdevice 400, the amplifier may be embodied as a commercially fabricatedamplifier, a multiple staged feedback amplifier, a single solid state orvacuum tube device or eliminated entirely. The latter possibility can beused when the switching device 400 has input requirements within thecapabilities of the generator 402.

When the switching device 400 conducts it shorts or removes ordissipates the charge built-up on the inherent capacitance 412 of theelectro-optical polarization modulating element 128. When the switchingdevice 400 is restored to the off condition, the capacitance 412 againcharges toward the high voltage at 428 through the resistance 416. Thischarging and shorting under control of the generator 403 creates thewaveform shown in FIG. 3 of the drawings.

The laser 104 in the FIG. 1 system produces synchronizing signals whichcommunicate along the path 132 to the DPGC 108. These signals arereceived at 408 in the FIG. 4 circuit by way of a RS 232 port or theIEEE standard port that is available on the Stanford DG 535 and similarother pulse generator apparatus usable at 402.

The signal across the capacitance of the electro-optical polarizationmodulating element 128 is present at the node 410 in FIG. 4. This signalcorresponds to the signal on the path 134 in FIG. 1 and controls thedegree of polarization modulation achieved and thereby proportions thelight dependent electrical signals flowing from the cameras 101 and 102.The representing electrical signals from the cameras 101 and 102respectively are coupled to the image processor boards 144 and 146 ofthe image processing system or microprocessor 106.

The laser 104 in the FIG. 1 system is selected to have a pulse durationthat is smaller than the time of signal propagation across a typicaldimension of the target 100 geometry. For target objects of the size ofan automobile, for example, the laser 104 is preferably of themode-locked Q-switched and cavity dumped yttrium aluminum garnet (YAG)type which is operated in the frequency doubling mode to produce a 532nanometer wavelength sequence of output pulses, pulses which arepreferably in the order of 50 picoseconds in pulse length. Such lasersare available, for example, from Quantek International Company as amodel YG 501C-10 laser. The output energy from the laser 104 iscommunicated along the paths 114 and 116 by way of the planar mirrors115 and 112 to the distal target 100 that is to be range mapped.

Optical energy returning from the distal target 100 is of greater crosssection as a result of scatter, as is indicated by the path 120 inFIG. 1. In addition to this divergence, the signals along the path 120are also displaced in time in accordance with the physical features anddimension of the target 100 and in keeping with the principle thatsignals returning from nearer portions of the target 100 involve ashorter distance of travel both to and from the target and thereby arereceived earlier in time than are the signals returning from moredistant portions of the target 100.

Light along the path 120 from the distal target 100 is concentrated bythe concave mirror 118 and imaged by the action of the convex mirror 122through the intermediate elements onto the receiving plane or retinas ofthe video cameras shown in FIG 1. Cameras such as the Xybion type ISS205 R3 may be used in the FIG. 1 system. Electrical signals originatingin the video cameras may be received by image processing circuit arrayssuch as are typically provided by Imaging Technology Incorporated modelFG-100-AT circuit boards located at 144 and 146.

The optical bandpass filter 124 in FIG. 1, as indicated above,preferably is arranged to favor the optical spectrum region in which thelaser 104 operates, that is, the spectral region adjacent a wavelengthof 532 nanometers; this filter may, for example, be embodied as a type03 IFS 008 filter which is available from Milles Griot Corporation. Thepolarizing filter 126 in FIG. 1 may be embodied as a CVI Corporationpart number CLPG 20 type filter.

An example of the electro-optical polarization modulating element 128 inFIG. 1 is comprised of a plurality of individual electro-optic crystals500 in FIG. 5 of the drawings. As indicated in FIG. 5, this arraypreferably consists of the plurality of electro-optic crystals,indicated at 502, 504, 506, and 508 with these crystals being disposedin an optical series and electrical parallel array configuration. Thisdisposition serves to distribute the polarization retardation angle andthereby reduce the required driver voltage, the signal applied at thenode 514 in FIG. 5. Each of the crystals 502-508 and so on in FIG. 5 mayconsist of a commercially available large aperture Pockels cell whichemploys a crystal of, for example, potassium di-hydrogen phosphate(KD*P). Such cells are available from Cleveland Crystal Company as theirmodel TX2650, for example.

In order to rotate the angle of polarization of light received along thepath 512 in FIG. 5, that is, light received from the mirror 118 of FIG.1, through an angle of ninety degrees, requires an electrical controlsignal of about two thousand four hundred volts be applied to a singlePockels cell crystal. With the four cell arrangement shown in FIG. 5,however, signals in the range of 600 volts are acceptable and are moreconvenient for generation in the signal generator circuit 108.

The rotated polarization output light beam of the array of crystals 500in FIG. 5 is shown at 510; this beam corresponding to the opticalsignals along the path 135 in FIG. 1. Polarization rotation in thecrystals 502, 504, 506, and 508 may, in fact, be accomplished by way ofeither the Pockels or Kerr effects, to achieve a fast time proportionedrotation of the plane of polarization in the individual cells, sucheffects being well known in the electro-optic art.

The light signal emerging from the FIG. 5 crystal array, the signalalong the path 135 is analyzed as to polarization in order to segregatenearer and more distal portions of the image received from the target100. This analysis results in intensity coding within the video cameraapparatus of FIG. 1.

FIG. 2 of the drawings shows the nature of the two signals accomplishedby the FIG. 1 electro-optical system. In the uppermost portion of FIG.2, the cameras 101 and 102 are indicated along with the polarizingbeamsplitter cube 210 and a representative optical system as depicted bythe lens 202. These elements receive signals from an exemplary targetrepresented by the pyramid 200. In the image received by the camera 102as shown at 220 in FIG. 2, the most closely disposed portions of thepyramid 200, that is the apex image at 222 and the edge linesapproaching the apex 222, appear in substantially normal form, however,as these lines approach the more distal portions at the base of thepyramid the intensity of their pixels decreases--as is represented bythe dotted lines in the image 220. Along the line 228, for example,pixel intensity decreases progressively from the portion at 244 to theportion at 242 and to the portion at 240. The differing intensities ofthe pixels along the line 228 therefore represent differing distancesfrom the lens 202 and the cameras 101 and 102 according to the codingscheme of the invention.

In a similar manner, the image received by the camera 101 is shown at230 in FIG. 2--with the difference that the relationship betweendistance and intensity is reversed from that in the image 220. In theimage 230, for example, the lines 234, 236, and 238 become of lowerintensity as they converge on the apex 232 with the line portion 240being of full intensity, the portion 242 of medium intensity and theportion 244 of lowest intensity.

It should be recognized that the FIG. 2 drawing at 220 shows theappearance of a target object which in real life has feature lines ofconstant intensity--since a target of this type is most useful indescribing the functions of the FIG. 1 system. In reality, however, atarget of varying feature line intensities is to be expected; thereforethe FIG. 1 optical modulation will tend to further modify these nonconstant feature line intensities according to their close or distallocation with respect to the FIG. 1 apparatus. The elected polarity ofthe optical modulation, that is whether the closely positioned or thedistally positioned portions of the target are to be decreased inintensity by the modulation will also affect the appearance of theimages received at the cameras with either polarity being usable ifaccommodated in other parts of the apparatus.

Considering briefly the theory supporting the FIG. 1 two camera system,there is an amplitude function A, B, for each pixel (i,j) in each of thetwo cameras which is equal to the product of a proportioning functionand a gain function:

    CAMERA #1A(t)*f(i,j)=sin.sup.2 [hV(t)]* f(i,j)             (1)

    CAMERA #2B(t)*g(i,j)=cos.sup.2 [hV(t)]*g(i,j)              (2)

where the complementary proportioning functions satisfy A+B=1 and wherepolarization angle is proportional to the voltage V and h is a constant.Such relationships are discussed in the text "Optics" published byAddison Wesley Inc., 1975 and authored by Hecht & Sajac. That A+B=1 isobvious because the two split beams conserve the energy in the incomingbeam. Different gain functions, f and g, characterize the lightdetection efficiency in the two separate cameras. The pixel indicesspecified by (i,j) allow for different gain constants for each pixel.

The cameras 101 and 102 integrate the light returned from multiple laserpulses which can occur in the time for one camera cycle--a time which istypically 1/30 of a second. FIG. 3 depicts the polarization modulatingsignals and operating times over one camera cycle. In FIG. 3 a series oflaser pulses 331, 332, and 333 are shown along with waveforms suitablefor operating the electro-optic polarization modulating element 128 inFIG. 1. The pulses applied to the element 128 are identified with thesymbols K=1, K=2, K=n in FIG. 3 and comprise several operating cycles asare indicated at 308.

In the FIG. 3 waveforms, the pulse envelopes 320 and 324 are determinedas to waveshape or curvature by the relative sizes of the resistor 416,the voltage at 428, and the capacitor 412 in FIG. 4. Actually, theprecise shape of these waveforms is not especially critical so long asthe shape is known and can be accommodated mathematically duringprocessing of the two camera received signals of the FIG. 1 system.

In the symbols shown in FIG. 3:

k=Pulse number in a given camera cycle.

N=Number of pulses or ramps per camera cycle.

Td=Width of time delay in seconds, corresponding to 326.

Tmax=Width of step or ramp in seconds, corresponding to 306.

u="Normalized Ramp" Time having values between zero and one, where avalue of one corresponds to Tmax seconds.

Vmax=the maximum applied voltage.

In order to simplify the following mathematical discussion, the timedelay, Td, is assumed equal to zero. When time delays, Td, are finite,range computations require the addition of a distance equal to (c*Td/2)to each input element, where c is the velocity of light.

With respect to the Kerr or Pockels cell polarizing beam-splitter cubeproportioning function, because the retardance of the linearpolarization is proportional to the voltage applied across thelongitudinal axis of the cell, the retardance angle satisfiesθ(u)=hV(u), where h is constant. The polarizing beamsplitter cube, 131in FIG. 1, splits the linearly polarized incoming beam into two beamsalong the paths 137 and 139 that are proportional to cos² (θ) and sin²(θ). These characterizations correspond to the squaring of the projectedamplitudes to determine intensity of the split beams emerging from thepolarizing beamsplitter cube.

With respect to incoming radiation, each pixel or picture element in thecamera image plane represents a solid angle subset from which the pixelintercepts the incoming beam. The energy collected by the (i,j) pixel isspecified by I(t;i,j) where the indices correspond to quantized polarangles to the reflecting surface element. If there is no surface elementto reflect light back into a pixel specified by indices (i,j) then thefunction I(t;i,j) would be zero throughout the camera cycle. Otherwise,the reflected light signals can be modeled by delta function pulses inthe relationship:

    I(t;i,u)=I(u,k;i,j)δ(u-2r(i,j)/c),                   (3)

where I(u,k;i,j) is the intensity of the kth reflected signal, r(i,j) isthe range or polar distance to the surface element reflecting light tothe (i,j) pixel, and c is the speed of light. Effectively, use of thedelta function model assumes that the emitted laser pulses are muchshorter than the width of the voltage proportioning signals applied tothe Pockels or Kerr cell array 128 in FIG. 1.

The light collected by each camera in each cycle is given by a sum of Nintegrals: ##EQU1##

In the following discussion the pixel indices are suppressed in thenotation and the measured quantities are understood to pertain to eachcamera pixel. The delta function model allows the followingsimplification: ##EQU2##

where I(k) indicates that the reflected energy in the differentreflected pulses may vary, whereas the range r is assumed to be constantover the camera cycle.

With respect to self-normalized ratios, note that the ratio,

    M1/M2=f/g tan.sup.2 [h*V(2r/c)],                           (8)

is independent of the sum of intensities of the reflected pulses. Thisindependence will be referred to as "self-normalization" and is in facta significant aspect of the FIG. 1 system since the cancellation ofabsolute intensity values is of tangible practical benefit in a physicalembodiment of the FIG. 1 system.

With respect to an equation solution for range maps, having a knowledgeof the voltage modulation V(u) or more specifically of its inversefunction, u=W[V], allows solving for the unknown range r in terms of theratio, S, of camera measurements (M1,M2) and known constants (f,g);i.e.,

    r=c*u/2=c/2W[sin.sup.=1 (S)/h]                             (9)

where

    S=(g*M1)/(f*M2))                                           (10)

Thus the range r(i,j) for each pixel is a function of the ratio of theamounts of light, Ml(i,j) and M2(i,j), gathered at corresponding pixelsin each camera. This relationship is an underlying concept of the FIG. 1system. With respect to self-normalized ratios, it is noted that thesums over I(k) in equations 6 and 7 are equal. We assume that the ranger remains a constant over the acquisition time for N pulses so that theratio M1/M2 is independent of the absolute intensity measurements madeby each camera in integrating N pulses as well as the absoluteintensities from the reflection of individual pulses. It is interestingto note that in the imaging and ranging system of the present invention,the integration of pixel intensities received during multiple laserpulse illumination of the target accomplishes a time averaging of thepixel pulses to improve the signal to noise ratio which also tends tofacilitate the conditions for self-normalization. This feature is basedon a mathematical approximation, referred to as the pulse setapproximation:

    M1/M2=Σ[M1(k)]/Σ[m2(k)]=Σ[M1(k)/M2(k)]/N (11)

    where

    rk=r constant range over one camera frame                  (12)

    M1(k)=f sin.sup.2 [h*V(2rk/c)]*I(k)                        (13)

    M2(k)=fcos.sup.2 [h*V(2rk/c)]*I(k)                         (14)

    N=number of pulses per frame                               (15)

This approximation provides a basis for integrating N pulses per cameraframe to compensate for a low reflected intensity obtained with a singlepulse per camera frame. It also provides a basis for using the ratio ofaccumulated averages M1 and M2, which are two direct and electricallybuffered measurements by the cameras 101 and 102 in one frame time toapproximate the average of N paired ratios <M1(k)/M2(k)>.

From a mathematical consideration the operating concepts of the FIG. 1ranging and imaging system and the FIG. 3 illustrated waveforms may beappreciated by considering that the proportioning of the return lightinto two cameras with the time dependent ramp measures thetime-of-arrival of the returning light and therefore the rangecoordinate r.

With respect to delay time and width of the proportioning voltagesignals applied to the electro-optic element 128 in FIG. 1, the timedelay Td at 330 in FIG. 3 and the ramp width Tmax at 306 may be softwarecontrolled or hardware controlled by circuit elements added to thedriver signal generator circuit 108 in FIG. 1. The delay time is largeenough to accommodate a time delay and avoid overlapping return signalsfrom different laser pulses 331 and 332 in FIG. 3.

It is also interesting to note that the depth of field for the presentinvention, that is the resolution, can be controlled electronically byway of adjusting the time delay and pulse duration of a signal voltageapplied to the electro-optical polarizing element 128 in FIG. 1. Thetime delay Td determines the minimum distance of the range fields, (cTd)/2. Both Td and Tmax determine the maximum distance c(Td+Tmax)/2. Thewidth of the range field equals (c*Tmax)/2. The delay time may be variedto optimize allocations of the range quantization and depth of field ofthe FIG. 1 system. The maximum resolution on a target surface occurswhen the range field matches the radial depth of the viewed targetsurface. This means, for example, smaller Tmax for smaller target depthsand larger Td for more distant targets.

According to the pixel intensity coding arrangement described above forthe most closely located and the most distally located portions of thetarget image, separate partial images of the target 100 are received inthe two cameras 101 and 102. Neither of these images is in fact acomplete representation of the target 100. To obtain a complete image ofthe target, however, the images communicated by way of the two cameras101 and 102 may be combined, such combination being preferably achievedby addition of the individual pixel intensities in the images formed bythe cameras 101 and 102 to determine the intensity of the target imagepixel. According to this combination arrangement, the intensity of atarget image pixel is determined by the square root of the sums of thesquares or the root mean of the square value of the two individualintensities resulting from the images of the cameras 101 and 102. Insome cases, it may desirable to compensate for differential gain betweenthe two cameras as discussed above.

It is also notable that the range mapping system of the presentinvention operates without regard to the absolute values of pixelintensity observed in the system cameras; that is, as has been shownabove herein, the absolute values of intensities mathematically canceland only the ratio of intensities appears in the output signal from thesystem.

The FIG. 1 system has been described in terms presuming the achieving ofprecise pixel to pixel registrations on the retinas of the cameras 101and 102 and the precise pixel to pixel comparisons of retina collecteddata. Although this is a somewhat idealistic basis for accomplishingimage pixel comparisons it may be achievable in some systems accordingto the invention. In many embodiments it may, however, be more practicalto accomplish the comparisons on a local average or local neighborhoodof pixels basis according to averaging arrangements that are known inthe art.

With respect to a further aspect of the invention the energy in theoutgoing laser pulse can be concentrated into a regular array of laserbeamlets. Such a beam array may be formed via a prism that is insertedin the optical path 114 in FIG. 1. This can be accomplished in a numberof ways, for example as explained in the hereby incorporated byreference publication by J. Taboada, "Laser Generated Beam Array forCommutation of Spatially Modulated Optical Signals," Proceedings 1986Society of Photo-Optic Instrumentation Engineers, Vol 698:198 (1986).The concentration of energy of the laser pulse into an array of beamletsmay provide several advantages in the system in FIG. 1. The array couldfacilitate the registration of the two cameras. This concentration ofenergy could be, for example, used to extend the range sensitivity inthe directions sampled by the beamlets.

The present invention therefore provides a number of improvements to theimaging and ranging art. Among these improvements are:

1. An electronic time scan with no moving parts.

2. Parallel optical scanning generated optical images and range mapswhich are fully registered. This provides capability for optimalparallel processing.

3. A time of flight approach to range mapping -- accomplished withouthigh frequency clocking as used by conventional light detection andranging systems.

4. A range map is achieved with a reflecting telescope.

5. Image amplification is achieved with a reflecting telescope.

6. The target illumination can use strong laser output.

7. The system achieves a new type of mono-axis ranging device usingparallel time of flight measurement and facilitates repetitiveaccumulation of signals via phase locked laser pulsing cycles.

8. A laser beam array as described in the 1986 publication above can beused to calibrate two cameras in order that corresponding imagescoincide at corresponding pixels in both cameras.

9. The projected laser beam can make use of structured encoding such asa spatial-temporal modulated beam or fringe arrays for increasedinformation yield.

10. The system employs one camera frame of time to achieve capture oftwo camera images.

11. In optimized arrangements a two camera system data capture may beachievable with the optical energy of a single laser pulse.

The invention therefore advances the art for obtaining a range image athigh speed and in real time for a scene presented to a camera basedvision system. The described system captures two-dimensional imageswhich contain range information encoded into a ratio of registered pixelpairs. A numerical map of the range distributed in perfect registry withan optical image is therefore a possible output arrangement for the datacollected by the system.

While the apparatus and method herein described constitute a preferredembodiment of the invention, it is to be understood that the inventionis not limited to this precise form of apparatus or method and thatchanges may be made therein without departing from the scope of theinvention which is defined in the appended claims.

We claim:
 1. The method for acquiring a range map of a distal three-dimensional object comprising the steps of:illuminating the distal object via a single optical aperture with short duration laser energy optical pulses; collecting target reflected laser energy optical pulses into a time encoding optical amplitude modulator; imprinting within said modulator a pattern of time dependent optical intensity modulation on said optical pulses; segregating the optical output of said modulator into two time dependent complementary amplitude optical portions; receiving said complementary portions onto the retinas of two optical-to-electrical energy transducer devices to generate a pair of electrical signals; determining from related pixel relative amplitudes of said electrical signals the relative arrival times of component portions of said reflected energy signals at said optical modulator; and generating from said relative arrival time determined component portions a range coded map image wherein arrival time and range are related by the speed of light.
 2. The method of claim 1 further including the step of generating from a combination of said pair of electrical signals an optical image of said object.
 3. The method of claim 2 wherein said related pixel relative amplitudes are local neighborhood of pixels determined.
 4. The method of claim 2 wherein said related pixel relative amplitudes are pixel to corresponding pixel determined.
 5. The method of claim 1 wherein said imprinting step is synchronized with and delayed from the occurrence of said laser pulses.
 6. The method of claim 1 wherein said imprinting step includes modulating the polarization of said laser energy reflected optical pulses in response to a predetermined ramp waveform.
 7. The method of claim 6 wherein said ramp waveform is mathematically monotonic in nature.
 8. A laser imaging and ranging system comprising the combination of:pulsed laser means for illuminating a distal three-dimensional target; first and second electrical signal generating camera members optically energized by the reflected laser light signals received from said distal target; modulated optical polarization means for segregating said reflected laser light signals proportionally into first camera received predominantly nearest target portion related components and second camera received predominantly distal-most target portion related components; and means for accessing the corresponding pixel point optically determined electrical signals from said first and second cameras and generating optical intensity coded signals therefrom.
 9. The imaging and ranging system of claim 8 wherein said optical polarization means includes one of a Pockels cell member and a Kerr cell member and electrical sawtooth waveform means for controlling the transmission of polarized optical signals therethrough.
 10. The imaging and ranging system of claim 8 wherein said camera members include solid state optical-to-electrical signal transducer members.
 11. The imaging and ranging system of claim 8 wherein said laser means and said reflected laser light signals communicate between said system and said target via a single optical aperture.
 12. Single optical aperture two camera imaging and ranging apparatus comprising the combination of:pulsed laser target illumination means communicating via said single optical aperture with a distal target; means including a pair of optical signal to electrical signal transducing camera members for generating first and second range coding distinguished pixel intensity array partial image electrical signal representations of said distal target; first and second electrical memory means for storing said first and second partial image electrical signal representations in pixel organized array; modulated optical path means including polarization means and optical beamsplitter means disposed in the reflected light path coupling said target said single aperture and said camera members for generating a pair of range-coded pixel intensity array partial optical images of said distal targets, one for each said camera member; modulation waveform generating electrical circuit means coupled with said modulated optical path means for controlling the modulation characteristics thereof in response to a predetermined target signal time of arrival discriminating mathematical function; and mathematical algorithm characterized electrical circuit means for computing the range determinative time of arrival coded intensity ratio of corresponding image pixels in said first and second electrical memory means stored partial images. 